Implement step 3. D of the line length determination algorithm

[TimJentzsch]

What problem does this solve or what need does it fill?

Step 3. D of the line length determination algorithm is missing in the implementation.

https://github.com/DioxusLabs/sprawl/blob/11701ee97d68b22a231cac081aab4308af136706/src/flexbox.rs#L354-L360

What solution would you like?

The step should be implemented according to the specification

What alternative(s) have you considered?

We could decide to divert from the spec in this case.

Additional context

Originally discussed in #39.